Preventing Galvanic Corrosion

By Choosing the Right Materials

Galvanic corrosion occurs when two dissimilar metals come into electrical contact with a conductive electrolyte, usually rainwater or groundwater. In this process, a metal atom is oxidized, during which it leaves its bulk metal after losing one or more electrons and is then transferred to another site. the site where the metal atoms lose electrons is called the anode, while the site where the electrons are transferred is called the cathode. (1)

The Galvanic Series is a list of metals arranges in order of their electrical potential in flowing seawater. The metals on the anodic, or active, end will corrode faster than the metals toward the cathodic, or passive, end. (2)

It is especially important to consider the potential of galvanic corrosion when choosing metal paneling, trim, and fasteners. When choosing materials, choose metals close together on the list, as metals close together generally do not have a strong effect on one another. The farther apart any two metal are on the list, the stronger the corroding effect on the more active metal. (3)

The Galvanic Series (4)

ANODIC - active

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Magnesium alloys

Zinc

Beryllium

Aluminum 1100, 3003, 3004, 5052, 6053

Galvanized steel

Cadmium

Aluminum 2017, 2024, 2117

Mild Steel (1018), Wrought Iron

Cast iron, Low alloy high strength steel

Chrome iron (active)

Stainless steel, 430 series (active)

Stainless steel 302, 303, 304, 321, 347, 410, 416, (active)

Nickel (resist)

Stainless steel 316, 317, (active)

Carpenter 20 CB-3 stainless (active)

Aluminum Bronze (CA 687)

Hastelloy C (active), Inconel 625 (active), titanium (active)

Lead-tin solders

Lead

Tin

Inconel 600 (active)

Nickel (active)

Brasses (naval, yellow, red, admiralty)

Copper (CA102)

Manganese bronze, tin bronze

Silicon bronze

Nickel silver

Copper-nickel alloy

430 stainless steel

Nickel (passive), aluminum, bronze

Monel 400, K500

Silver solder

Nickel (passive)

Chrome iron (passive)

302, 303, 304, 321, 347, stainless steel (passive)

316, 317, stainless steel (passive)

Carpenter 20 CB-3 stainless (passive), Incoloy 825

Nickel-molybdeum-chromium-iron alloy (passive)

Silver

Titanium and titanium alloys

Graphite

Zirconium

Gold

Platinum

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Cathodic - passive

Minimizing Galvanic Corrosion

Use metals that are not dissimilar

Prevent dissimilar metals form becoming electrically connected by water

Keep small anodes from contacting large cathodes. The rate of corrosion depends on the surface area of the anode with respect to the cathode. The smaller the surface area of the anode relative to the surface area of the cathode, the more concentrated the flow of electrons at the anode and the faster the rate of corrosion. The larger the anode's surface area in relation to the cathode, the more spread out of the flow of electrons and the slower the rate of the anode's corrosion. (5)

The application of a protective metallic coating, known as a sacrificial coating, can provide galvanic protection to the base metal when the coating is measurably more anodic than the base metal. galvanic corrosion will take place with the anodic material when the base material is exposed. The extent to which a sacrificial coating can continue to protect the base metal is directly related to the thickness of the coating. (6)

Metallic coasting that are not sacrificial, as well as paint coatings, plastic, or other non-metallic barriers can also significantly reduce galvanic corrosion. however, when using a paint coating, it is important to realize that if the base metal becomes exposed through a small scratch in the paint, the base metal could rapidly corrode if it becomes the anode in a reaction with a nearby dissimilar metal with a large surface area. (7)

Preventing Corrosion in Fasteners

Galvanic corrosion is obviously a concern in the use of metal fasteners such as bolts, screws, and welds. Because fasteners have a much smaller surface area than the materials they fasten, fasteners that take on the role of the anode will be at risk of rapid corrosion and thus should be avoided. For example, zinc-coated fasteners should only be used to connect steel coated with aluminum, zinc, and galvalume, as these are very close on the Galvanic Series and are not generally at risk of corrosion when placed together. On the other had, zinc-coated or aluminum-coated fasteners should not be used to attach copper or stainless-steel panels.

To minimized the risk of galvanic corrosion of fasteners, match the surface metal on the fastener with that on the metal it will fasten. (8) The most desired combination of large anode with small cathode; in other words, fasteners such as bolts and screws should be made of the metal less likely to corrode, or the more cathodic.

The following chart can be used to guide the selection of fasteners based on galvanic action: (9)

Corrosion of Panels and Trim in Contact with Treated Wood

Do not allow aluminum, aluminum-coated, and galvalume-coated panels and trim to come into direct contact with wood preservatives containing copper, mercury, or fluorides. Avoid direct contact between bare metal panels and treated lumber where condensation will frequently form on the metal surface in contact with the lumber, and where the wood treatment is more noble than the metal surface. Use an appropriate barrier to separate metal panels and treated lumber. (10)